Luke B. Roberson, Mark A. Poggi, Janusz Kowalik, Greg P. Smestad, Lawrence A. Bottomley, Laren M. Tolbert

Correlation of morphology and device performance in inorganic-organic TiO₂-polythiophene hybrid solid-state solar cells

Coordination Chemistry Reviews 248 (2004) 1491–1499

Flat solid-state polymer photovoltaic (PV) cells were constructed using undoped poly(3-undecyl–2,2'-bithiophene) (P3UBT) and flat titanium dioxide (TiO₂) films prepared using a sol–gel technique. Layers and interfaces were studied using AFM, SEM, EDX, XPS, and optical microscopy to determine the impact of the interfacial boundary morphology on the PV device performance. Friction mapping and surface roughness measurements of the fluorinated tin oxide (SnO₂:F) conductive glass, TiO₂, and P3UBT surfaces show a distinct difference in surface contours enabling a better understanding of light absorption as well as charge separation and injection by the polymer. It was found that each working layer in the photovoltaic device exhibited a planarizing effect, where each successive film was slightly smoother than the underlying layers. TiO₂ films greater than 70 nm in thickness fractured during sintering and all films failed following sintering times exceeding 1 h at 450 °C. Fracture of the TiO₂ film resulted in exposure of the underlying SnO₂:F glass and short-circuited solar cells. The film thickness varied with viscosity of the sol–gel and could be controlled by the synthesis conditions and age of sol–gel used. Additionally, the hydrophilic TiO₂ surfaces readily absorbed water when exposed to ambient conditions thereby altering the interfacial boundary and degrading PV cell performance. Spin-coated P3UBT deposited at high concentrations and low spin speeds was found to deposit a layer a few nanometers thick containing polymer aggregates between 5 and 30 μm in diameter, which resulted in a solar cell with J_sc = 2 μA/cm² and V_oc = 0.2 V. The optimized spin- and cast-coated P3UBT layers were used with the best TiO₂ films to create flat solid-state photovoltaic inorganic–organic hybrid cells with J_sc = 55 μA/cm² and V_oc = 0.5–0.7 V. Films with fewer defects and aggregates yielded better PV device performance.

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Cited Articles

1. Franta D., Ohlídal I., Klapetek P., Pokorný P.,
   Characterization of the boundaries of thin films of TiO₂ by atomic force microscopy and optical methods,
   Surface and Interface Analysis 34 (2002) 759–762